A hermetic compressor used, for example, in household freezer-refrigerator is recently demanded to be lower in power consumption. A conventional hermetic compressor is improved in efficiency by modifying the outline shape of piston to reduce frictional loss between piston and cylinder. Such technology is disclosed, for example, in PCT International Publication WO02/02944A1.
This conventional hermetic compressor is described below while referring to the drawings.
FIG. 6 is a longitudinal sectional view of conventional hermetic compressor, and FIG. 7 is a perspective view of piston described in the publication used in the conventional hermetic compressor.
In FIG. 6 and FIG. 7, a motor element 4 and a compressor element 5 are accommodated in a hermetic container 1. The motor element 4 consists of a stator 2 having a winding 2a, and a rotor 3. The compressor element 4 is driven by the motor element 4. Oil 6 is stored in hermetic container 1.
A crank shaft 10 included in the compressor element 5 has a main shaft 11 mounting the rotor 3, and an eccentric shaft 12 formed eccentrically to the main shaft 11. Inside of the main shaft 11, an oil pump 13 is provided to be open in the oil 6. A block 20 has a cylindrical cylinder 21, and a bearing 22 for supporting the main shaft 11. The block 20 is disposed above the motor element 4. A piston 30 is inserted reciprocally and slidably in the cylinder 21 of the block 20, and is coupled to the eccentric shaft 12 by way of a linkage part 41.
As shown in FIG. 7, the piston 30 is composed of top surface 31, skirt surface 32, and outer circumference 33. The outer circumference 33 includes a seal surface 34, at least two guide surfaces 35, and removing part 36. The seal surface 34 is formed to contact tightly with the inner circumference of the cylinder 21. At least two guide surfaces 35 are formed to contact with part of the inner circumference of the cylinder 21, and is extended almost parallel to the moving direction of the piston 30. The removing part 36 does not contact with the inner circumference of the cylinder 21.
This prior art is characterized by that the angle formed by the line linking central axis 37 of cylindrical piston 30 and boundary edge 35a of guide surface 35 in radial direction of piston 30, and the line linking central shaft 37 and boundary edge 35b of guide surface 35 in radial direction of piston 30 is 40 degrees or less, or preferably 30 degrees or less.
In the conventional hermetic compressor having such configuration, the operation is explained below.
During operation of hermetic compressor, the piston 30 back and forth in the cylinder 21. When the piston 30 is in a bottom dead center, part of skirt side of the piston 30 protrudes from the cylinder 21. When the piston 30 gets into the cylinder 21, it is guided by the guide surface 35 and smoothly moves into the cylinder 21. The sliding surface formed by the inner circumference of cylinder 21 and outer circumference of piston 30 is decreased by the removing part 36 of piston 30 and is hence reduced in sliding resistance, so that the sliding loss can be decreased.
When moving from the bottom dead center to a top dead center in compression stroke, the top surface 31 of piston 30 receives compressive load of refrigeration gas. At this time, the crank shaft 10 is pushed by force toward the anti-piston direction by way of the connecting rod 41, and the crank shaft 10 is bented. As a result, a strong force acts to incline the piston 30 in vertical direction.
In the conventional configuration, however, inclination of piston 30 on cylinder 21 in vertical direction is regurated only by the small interval from the edge of top surface 31 of piston 30 to edge of seal surface 34, and by a gap of outer circumference 33 of piston 30 and cylinder 21. Accordingly, the piston 30 is inclined largely, and the amount of refrigerant gas leaking from the top dead center to the bottom dead center of piston 30 is increased through the gap expanded by increase of slope angle of pistons. As a result, the refrigeration capacity of hermetic compressor is lowered.
Along with increase of slope angle of piston, surface pressure increases in boundary edges 35a, 35b of guide surface 35 of piston 30, and local wear is likely to occur. As a result, the reliability as hermetic compressor is lowered, and the efficiency is also lowered.
These problems are particularly manifest when R600a is used as refrigerant because, generally, the outside diameter of piston 30 is larger, and the refrigerant is likely to leak out. Hence, in the hermetic compressor using R600a as refrigerant, efficiency is lowered significantly.